1. Field of the Invention
The present invention relates to an aluminum support for a lithographic plate, which support has the surface roughened by an electrolytic etching. In particular, the present invention relates to an aluminum support for a lithographic plate satisfying the following conditions:
(a) an arithmetic mean of the pit diameters of the electrolytically etched support is 4 .mu.m or less, PA0 (b) a difference between an arithmetic mean (D.sub.L) of the maximum pit diameter of the support in the rolling direction and an arithmetic mean (D.sub.LT) of the maximum pit diameter of the support in the direction perpendicular to the rolling direction is larger than 10% of the maximum pit diameter (a larger one of D.sub.L and D.sub.LT), PA0 (c) the number of pits detected with a contact profile recording instrument used for surface roughness (hereinafter referred to as "a profilometer") using a stylus having a tip radius of 1 .mu.m is at least 200/mm, and PA0 (d) an average centerline roughness is 0.2 .mu.m to 1.0 .mu.m. PA0 (a) an arithmetic mean of pit diameters is 4 .mu.m or less, PA0 (b) a difference between an arithmetic mean (D.sub.L) of the maximum pit diameter of the aluminum support in the rolling direction and an arithmetic mean (D.sub.LT) of the maximum pit diameter of the aluminum support in the direction perpendicular to the rolling direction is larger than 10% of the maximum pit diameter (either a larger one of D.sub.L and D.sub.LT), PA0 (c) the number of pits detected with a profilometer using a stylus having a tip radius of 1 .mu.m is at least 200/mm, and PA0 (d) an average centerline roughness is 0.2 .mu.m to 1.0 .mu.m.
2. Prior Art
Aluminum sheets have been widely used as supports of lithographic plates. The surface of the support is roughened or grained in order to facilitate the adhesion between the support and the photosensitive layer and to impart water-retaining properties to a non-image area.
The graining is conducted by a mechanical method such as a sandblasting method, ball graining method, wire graining method, brush graining method wherein a nylon brush and an aqueous abrasive slurry are used, or a method wherein an aqueous abrasive slurry is sprayed on the support surface at a high speed, or by a chemical method wherein the support surface is roughened with an etching agent comprising an alkali, an acid or a mixture of them. Further, an electrochemical graining method is described in Japanese Patent Unexamined Published Application (hereinafter referred to as `J. P. KOKAI`) Nos. 54-146234 and 48-28123, a combination of the mechanical graining method with the electrochemical graining method is described in, for example, J. P. KOKAI No. 53-123204 and a combination of the mechanical graining method with the chemical graining method wherein a saturated aqueous solution of an aluminum salt of a mineral acid is used is described in U.S. Pat. No. 4,242,417.
Among the above-described surface-roughening methods, the electrolytic roughening method is preferred, because the pattern of the roughened surface can be easily controlled and the fine roughened surface can be formed.
Literatures which disclose the characterization of the surface pattern have been known. For example, U.S. Pat. No. 4,301,229 describes the cumulative frequency distribution of the pit diameter and the average roughness of the centerline; U.S. Pat. No. 3,861,917 describes the depth of the rough surface; Canadian Patent No. 955,449 describes the height and diameter of the peaks on the rough surface; West German Patent No. 1,813,443 describes the difference in heights of the peaks on the rough surface; and J. P. KOKAI No. 55-132,294 describes the average depth (the average roughness determined with a profilometer).
Although various parameters are defined in these techniques, both of the printing durability and the stain-proofness of the non-image area cannot be obtained at the same time by them.
U.S. Pat. No. 4,581,996 discloses a lithographic susbstrate whose electrolytically grained surface is defined by specific six parameters including a distribution of pit diameters, a pit diameter directionality, a centerline average roughness and a roughness directionality.
However, this specification is silent on the relation between (1) the printing durability and stain-proofness of the non-image area and (2) the parameters.